1. Field of the Invention
The present invention relates to the elimination of noise induced by antenna switching in a radar apparatus constructed to perform switching between a plurality of receiving antennas by using a switch.
2. Description of the Related Art
Such techniques as DBF (Digital Beam Forming) and phase monopulse that use a plurality of receiving antennas are known for determining target bearing. Further, in Japanese Unexamined Patent Publication No. 11-160423, for example, it is described that the number of expensive high-frequency components can be reduced by employing a configuration for sequentially selecting a plurality of receiving antennas by switching from one antenna to another.
In a radar apparatus having a configuration, such as that shown in FIG. 1, in which a plurality of receiving antennas are sequentially selected by switching from one antenna to another, signal power leaks from the transmitter side to the receiver side of the high-frequency section in the form of a DC current and, as the DC current value differs from one channel to another, a rectangular wave such as shown in part (A) of FIG. 2 is generated at the output of a mixer 7. This rectangular wave causes transient response noise, such as shown in part (B) of FIG. 2, due to the characteristics of a filter at a subsequent stage. When the filtered signal shown in FIG. 2(B) is Fourier transformed, a noise peak appears at the frequency position corresponding to its frequency component; if this noise peak is close to the peak frequency occurring due to an echo signal from a target, there arises the problem that it becomes difficult to analyze the echo signal. For example, if the noise frequency appears in the vicinity of the peak frequency of an echo signal from a target located at a short range, it becomes difficult to discriminate the peak occurring due to the short-range target, and the short-range detection performance thus degrades.